1. Field of the Invention
This invention pertains generally to chemical treatment systems, and more particularly to a modular point of use chemical treatment system that permits individual treatment of specific chemical process mixtures.
2. Description of the Background Art
Several point-of-use chemical treatment systems are commercially available which utilize thermal, wet scrubbing, dry scrubbing, catalytic, or plasma technologies. However, the current implementations of these technologies tend to suffer from poor economic feasibility or low chemical treatment efficiency due to the need for multiple and simultaneous treatment of specific chemical process mixtures. Many equipment manufacturers accommodate the need for process specific treatment solutions by offering several different products, based upon different technology implementations, that provide solutions for individual specific process mixtures and not for multiple process mixtures. Because of the vast number of different process streams that may need to be treated, a commensurate number of standalone abatement technologies and systems have been developed, most notably those that are described in the following discussion.
Thermal reactors utilize a variety of means to heat chemical process mixtures to high equilibrium temperatures. In addition, reagent gases are added to promote specific reaction chemistries; for example, oxygen or air can be added to promote oxidation. The chemical process mixture can be heated upon contact with a hot surface or through mixing with hot gas. However, hot surface reactors are greatly susceptible to corrosion failure and material accumulation leading to clogging. Common hot gas reactors utilize combustion to heat and react the chemical process mixture in the gas phase. Combustion systems are extremely sensitive to fuel gas and oxidizing gas control and mixing. Corrosion and material accumulation on injection nozzles and igniters result in degraded performance of these systems unless frequent maintenance is performed. A significant expense and liability associated with combustion systems is the piping of fuel gas (specific to the combustion reactor) throughout the manufacturing facility to permit equipment installation.
Glow discharge plasmas are capable of depositing much of their applied power into the gaseous medium to achieve a high percentage of ionization or dielectric breakdown. Such devices operating with electrode potentials of several hundred volts can deposit several kilowatts of power using high frequency alternating current. Consequently, plasma processes creating energetic electrons, ions, reactive neutrals, and radical species can be promoted in these devices. Complex and diverse plasma chemistries can be conducted, but only at low relative pressures, typically 1 millitorr to 10 torr. Coupling power into the gas stream becomes difficult above these pressures resulting in non-uniform dielectric breakdown and collapse of the plasma region to the electrode (inductively coupled) or electrodes (capacitively coupled) of these devices. Because low pressure operation requires pumping systems, the chemical throughput is directly dependent upon both the scale of the pumping system and the ability to apply high frequency power. In addition, changes to the electrode material or geometry occur as a result of corrosion and material accumulation. Such changes interfere with power coupling to the plasma as a result of detuning and energy dissipation by deposited materials. Chemical processing applications have been limited to low flow gas processing such as chemical vapor deposition, gas phase chemical etching, gas phase spectroscopy, or surface treatment of materials such as fibers and films.
Microwave plasmas are also limited to low pressure operation. As pressures approach 100 torr, the inability to propagate and tune coherent waves limits the ability to deposit energy within a resonant cavity resulting in plasma collapse to the cavity surface. As low pressure operation requires a pumping system, the chemical throughput is again directly dependent upon both the scale of the pumping system and the ability to couple high frequency power. In addition, changes to the cavity material or geometry occur as a result of corrosion and material accumulation. Such changes interfere with power coupling to the plasma by changing the modes of TE and TM fields propagated within the cavity.
Silent discharge plasmas are capable of operating at relatively high gas pressures, typically above one atmosphere. Dielectric breakdown of gases between electrodes with a relatively large separation can be achieved using high voltage potentials. However, such devices require a dielectric barrier between the electrodes and the gas stream to prevent collapse of the plasma due to arcing. The electrical resistance of the dielectric barrier enhances capacitive coupling of power to the plasma, but this resistance also limits the current flow through these devices. While silent discharge plasmas are capable of promoting complex chemistries, plasma chemical activity diminishes dramatically as a function of increased gas flow. Minimizing the dielectric strength of the electrode barrier by placing dielectric material throughout the discharge gap has provided enhanced power deposition, but the complexities of surface reaction chemistries as well as physical fouling are increased. Thus, catastrophic arcing remains the principal failure mode and limiting factor in power deposition within these plasma devices.
Catalytic reactors require moderately high temperatures to promote chemical reactions and desorb reaction products from the catalyst surface. The process gas is preheated by either hot surface contact or gas phase combustion. Therefore, maintenance considerations for the preheating section of the system are similar to those for thermal reactors. Additionally, reaction products can poison the catalyst surface by forming a physical barrier or by chemically bonding to the surface. Where reaction products remain as solids even at high temperatures, physical poisoning occurs. Additionally, locations where reaction products are highly oxidizing or capable of forming stable salts upon reaction with the catalyst, chemical poisoning occurs.
Wet scrubbers require relatively meager amounts of energy for operation since their functionality extends from the inherent chemical affinity between the scrubbing solution and the process gases being treated. Both chemical reactivity and solubility are principle parameters effecting the efficiency and capacity of the scrubbing process. Mass transfer is the physical parameter most important to the efficiency of the scrubbing process. Two primary distinctions are important to properly applying wet scrubbing technology: (1) simple liquid with or without additional reagents, and (2) low energy or high energy. The first distinction dictates the ability to satisfy the chemical parameters for scrubbing when treating specific gases. The second distinction dictates the ability to ensure the requisite physical contact between gaseous or particulate matter and the scrubbing liquid to effect removal. Treatment requirements that create difficulties for wet scrubbers are hazardous process mixtures comprising both gaseous and particulate matter where some of the gases are largely insoluble in the scrubbing liquid such that insufficient reaction occurs with the specific reagents, and particulate having diameters in the nanometer to submicron range is thereby formed prior to, or during, the scrubbing process.
Dry scrubbers vary greatly with regard to energy requirements which range from a simple fixed bed adsorption system having low power consumption levels, to a heated reagent bed having a power consumption which approaches that of a catalytic system. Pressure swing or temperature swing adsorption systems have not been feasible for treating complex and reactive gas mixtures and are, therefore, not discussed. Primary distinctions between various dry scrubbers are physical adsorption, and chemical adsorption. Physical adsorption refers to condensation or molecular trapping processes which occur within the requisite material matrix. Chemical adsorption refers to a combination of physical adsorption and surface chemical reaction processes which bind the molecules of concern to the requisite material matrix. Often chemical adsorption systems require added thermal energy to promote the necessary surface reactions. In both types of dry scrubbers the common concerns are premature clogging of the material matrix by particulate formed upstream, and a limited capacity which requires regular replacement and disposal of the material matrix.
As can be seen, therefore, a number of abatement options are available for the process streams in semiconductor wafer manufacturing. However, the equipment available is generally constructed such that its abatement capabilities are limited to one or two process streams. Industry economics (both cost and space availability) largely dictate that a typical treatment system must simultaneously treat at least four process streams to be feasible. Typical semiconductor wafer manufacturing equipment can simultaneously carry out multiple chemical processing operations. Thus, a single treatment system that is optimized for one or two chemical processes suffers the loss of chemical treatment efficiency and severe maintenance requirements from exposure to the additional chemical process mixtures. Furthermore, because many of the materials requiring treatment are pyrophoric or flammable, serious risk of explosion and fire is associated with poor treatment efficiencies or equipment bypass during unplanned maintenance. Currently, the semiconductor wafer manufacturer is forced to make a compromise in selecting treatment equipment and accept the inherent risk, thus creating a need for a system that is economic, reconfigurable and reliable. The present invention satisfies that need, as well as others, and overcomes deficiencies in current treatment systems.